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Cable Guardrail Questions

Question
State NE
Description Text

The new in-line cable end treatment requires post 3 through 7 to be spaced @ 16'. What is the offset to a fixed object in this area?

 

When we design a long run of guardrail in the past we have used an intermediate anchorage section. Is this still necessary?

 

If so, is there a design for the new in-line intermediate anchorage section?

 

The spacing in front of a 1.5:1 slope requires 4' post spacing. Is it acceptable to have 16' post spacing then 4' spacing?

 

Or, is there a suggested length of transition of 8' post spacing?

 

Have you been able to run a simulation when our slope is 2:1, with a 2% lane and 4% shoulder slopes? I think this will keep the front tire on the slope and not require the 4' post spacing.

Keywords
  • Guardrail
Other Keywords none
Date July 30, 2010


Response
Response

The new in-line cable end treatment requires post 3 through 7 to be spaced @ 16'. What is the offset to a fixed object in this area?

**A 2000P pickup truck was crash tested at the length-of-need of the end terminal at the TL-3 conditions of NCHRP Report No. 350. The vehicle impacted post no. 3 which was 15 ft downstream from the upstream steel anchor post. For this crash test, the working width was reported to be approximately 84 in. when using a 254-ft long installation.

 

**Please note that the target impact angle for this test was 20 degrees, as required by NCHRP Report No. 350. The new MASH guidelines now utilize an impact angle of 25 degrees. With higher impact angles, one would expect higher angle loading and slight increases in anchor movement, thus resulting in greater barrier deflection and working width near the system ends.

 

When we design a long run of guardrail in the past we have used an intermediate anchorage section. Is this still necessary?

**As noted above, the test installation was 254 ft long. For longer test installations than denoted above, dynamic barrier deflections and working widths would be expected to increase.

 

**A prior Pooled Fund R&D program resulted in the successful development, testing, and evaluation of three alternative anchor systems in lieu of the large cast-in-place reinforced concrete anchor blocks. However, the R&D program did not evaluate changes in anchor spacing. As such, we would recommend that NDOR continues to utilize an anchor spacing equal to or smaller than that currently specified, especially since barrier deflections and working widths could be greater with the use of the alternative anchor options.

 

If so, is there a design for the new in-line intermediate anchorage section?

**The alternative anchor options were developed for terminating and anchoring the ends of the three cables. I am unclear as to the difference between end anchor hardware and the anchor hardware used at intermediate anchor sections. Please forward those details to us for review as I am unaware of prior crash tests performed to evaluate the safety performance of the overlapped cables with two intermediate anchor sections crossed in opposite directions.

 

The spacing in front of a 1.5:1 slope requires 4' post spacing. Is it acceptable to have 16' post spacing then 4' spacing?

**The SdDOT three-cable guardrail to W-beam transition utilizes a cable barrier with 16-ft post spacing that transitions into a cable barrier with 4-ft post spacing in advance of the BCT W-beam terminal. No intermediate post spacing was integrated into this original SdDOT design. More than 60 ft of cable barrier with 4-ft spaced posts was used to prevent pocketing near the BCT end. No testing was performed upstream of the 4-ft post spacing design. However, I do not believe that the reduction in post spacing would create a significant pocketing concern for large vehicles or penetration concern for small cars when used in combination with the standard cable hook bolt.

 

**For the three-cable barrier with 4-ft post spacing in front of a 1.5:1 fill slope, MwRSF performed a 2000P crash test according to the TL-3 conditions of NCHRP 350. An 820C small car test was not performed nor deemed necessary by the MwRSF team. The successful 2000P crash test resulted in nearly 125 in. of dynamic deflection when placed 4 ft from the slope break point, thus resulting in the vehicle extending nearly 6 ft off of the slope. The vehicle's lateral extension off of the slope further accentuated the barrier deflections observed in the 2000P test.

 

**TTI crash tested a 3-cable barrier on level terrain with a 16-ft post spacing at TL-3 of NCHRP 350. This testing resulted in 3.4 m (134 in.) of dynamic deflection, which was slightly larger than the deflection observed above in the ditch. Since it is uncertain where the 4-ft post spacing will end w.r.t. the ditch start/finish, it would be reasonable to expect the 4-ft spacing to overlap regions of level terrain. When the 4-ft post spacing is installed on level terrain, dynamic deflections would likely be reduced below 125 in.

 

**Although it would not be deemed necessary at this time, one may consider the use of 4 or 5 spans with posts spaced on 8 ft centers prior to reaching the 16-ft post spacing region.

 

Or, is there a suggested length of transition of 8' post spacing?

**See comments noted above.

 

Have you been able to run a simulation when our slope is 2:1, with a 2% lane and 4% shoulder slopes? I think this will keep the front tire on the slope and not require the 4' post spacing.

 

**No work on this project has been performed. This work was included in a Pooled Fund study that was not funded in the Year 21 final program. I will copy this request to John Reid and Bob Bielenberg to determine what level of effort would be required to conduct this specific request.

Date August 25, 2010


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